usb: gadget: f_midi: use flexible array member for gmidi_in_port elements

Reduce number of allocations, simplify memory management and reduce
memory usage by stacking the gmidi_in_port elements at the end of the
f_midi structure using a flexible array.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>